Ultra-thin carbon-nanotube films can act as thermoacoustic loudspeakers

Evidence

The next great thing in loudspeakers could be a development made possible by carbon nanotubes. Shoushan Fan and his research team at Tsinghua University in Beijing developed a thin-film thermoacoustic loudspeaker using embedded carbon nanotubes (CNTs). The Fan design is in fact a distant cousin of lightning-produced thunder. The CNTs are incredibly small in diameter, just a few tens of Angstroms across, but possibly thousands of times longer. If the nanotubes are embedded in a very thin (tens of nanometers, one 1000th the width of a human hair) membrane, the membrane becomes quite conductive to electricity, which means relatively large currents can be supported. There is very little inductance, which is the electrical analog to mass and inertia. Inductance complicates the relation between sound amplitude and current, and its absence is a big advantage. There is electrical resistance in the membrane, so it heats up as current is put in. The adjacent air takes up the heat very efficiently; the membrane stays relatively cool. It is so thin that as soon as the current stops, the membrane cools to ambient temperatures. … The thin film heats air in proportion to the current passed through it. If freestanding, both sides of the film are in phase, making it a monopole source. The film does not move during sound production and can also be attached to any insulating surface that does not accept heat readily. A key point is that the films can be made cheaply and quite large; they are also stretchable and flexible. Whole walls and even clothes could become loudspeakers. The Beijing group has already demonstrated a waving flag playing music. — Eric J. Heller, Why You Hear What You Hear- An…, 732

Categorization

Category: Causal Relationships

• XREF: Directly connects to the lightning-thunder mechanism in section 7.9; thermoacoustic transduction is the same sudden-heating acceleration principle.
• QUESTION: Worth chasing: Fan’s thermoacoustic films have no moving parts and bendable form factors — why haven’t they displaced conventional speakers commercially?
• SEED: Could scaffold an article on ‘speakers of the future’ — solid-state, wall-sized, wearable sound sources and the physics making them possible.

Related

• Ultrathin thermoacoustic films can turn entire walls and clothes into loudspeakers (similarity 0.85)